The invention relates to a process for operating an electronically commutated electric motor whereby a standard value signal is readied for the commutating position of the electric motor and the windings of the electric motor are acted upon by current such that the rotor of the electric motor is positioned on the commutating position specified by the standard value signal, and whereby a measuring signal for the position of the rotor of the electric motor is recorded.
The invention moreover relates to an electric motor drive with an electric motor which is connected with the a.c. system through a frequency converter, whereby the frequency converter is connected with a control apparatus which has a terminal connection for a standard value signal for the commutating position of the rotor of the electric motor, and whereby a position measuring apparatus having a position sensor for measuring the rotor position is provided.
An electric motor drive of the type mentioned at the beginning is already known from practice which has two position sensors for measuring the rotor position of the electric motor. The position sensors are generally constructed as rotary transducers which are rigidly coupled with the axis of the rotor of the electric motor. The measuring signal outputs of these position sensors are in any given case connected with an input of a comparison device by means of which the measuring signals of the two position sensors can be compared with each other. The output of the comparison device is connected to a shut off apparatus which interrupts the current feed to the electric motor if a difference between the two measuring signals of the position sensors is detected which lies outside the specified tolerance range. The electric motor drive makes possible a secure monitoring of the motion of the rotor of the electric motor. The electric motor drive, however, has the disadvantage that position sensors are comparatively costly and expensive. In addition, the two position sensors mechanically connected with the electric motor have a certain space requirement.
An electric motor drive of the type mentioned at the beginning which has only one position sensor for measuring the rotor position is also already known from practice. The measuring signal output of the position measuring apparatus is connected with the inputs of two monitoring devices which monitor the measuring signal of the position sensor independently of each other. By means of the monitoring devices, certain typical errors of the position measuring device are recognizable, as for example the occurrence of a cable disruption, an unplausible position measuring signal or an electrical output voltage applied to the measuring signal output of the position measuring device which lies outside an allowable tolerance range. If one of the two monitoring devices detects such an error, the electric motor is shut off. The costs for a second position measuring device can indeed be saved by the two monitoring devices, but the monitoring devices are also comparatively costly and expensive. It is moreover unfavorable that the error recognition functions of the monitoring devices must be specially harmonized with the position measuring device so that, for electric drives with different position measuring devices, different monitoring devices are also necessary which makes above all the development and bearing maintenance costs for the monitoring devices greater.
From DE 43 22 146 A1, an electric motor drive is also already known where the intermediate circuit voltage is reducible by interposing a transformer. With reduced intermediate circuit voltage, the rotational speed of the electric motor is determined by evaluation of rotation frequency and compared with a specified threshold value. When this threshold value is exceeded, the electric motor is shut off. The electric motor drive has, however, only a comparatively low operating safety since faulty positioning of the rotor only leads to shutting off the electric motor if the rotor rotational speed exceeds the rotational speed threshold value. By utilizing the rotating field, a mechanical component for the rotational speed measuring facility can indeed be saved, but the measurement of the rotating field is nonetheless still associated with a certain expense. It is moreover not beneficial that the electric motor, due to the reduced intermediate circuit voltage, only enables a comparatively low rotational speed and a low acceleration.
From DE 43 30 823 C2, a regulated electric motor drive is already known where the rotational speed of the electric motor is monitored twice. In a first monitoring channel, the rotational speed of the electric motor is ascertained with a rotational speed sensor coupled with its rotor axis and the electric motor is shut off when the measuring signal of this rotational speed sensor exceeds a specified threshold value. The electric motor is regulated by means of a governor which has a cascade structure whereby a rotational speed regulator is connected in series in front of a current regulator and a position regulator is in turn connected in series before this. In a second monitoring channel, the course of the current over time is monitored in at least one phase supply lead and the rotational speed of the motor is ascertained from the course of the current. If the rotational speed signal thus ascertained exceeds a specified threshold value, the electric motor is shut off. Even this electric motor drive has only a comparatively low operating safety as faulty positioning of the rotor even here only leads to shutting the electric motor off when a rotational speed threshold value is exceeded.
There thus exists the object of creating a process for operating an electronically commutated electric motor which makes possible monitoring of motion and consequently a safe operation of the electric motor in a simple manner. There moreover exists the object of creating a simply constructed electric motor drive which makes possible a secure, motion-monitored operation of the electric motor.
This objective is accomplished in accordance with the present process in that a standard value signal for the commutating position of the rotor of the electric motor is readied and the windings of the electric motor are subjected to current such that the rotor of the electric motor is positioned on the commutating position specified by the standard value signal, that a measuring signal for the position of the rotor of the electric motor is recorded and compared with a standard value signal for the commutating position, and that current to the windings of the electric motor is interrupted when the position measuring signal and/or the standard value signal for the commutating position exceed a specified maximum value, and/or if the differential signal between the position measuring signal and the standard value signal for the commutating position lie outside a specified tolerance range.
The aforementioned object is accomplished in relation to the electric motor drive in that the electric motor is connected to the a.c. network through a frequency converter, and in that the frequency converter has a control apparatus which has a terminal connection for a standard value signal for the commutating position of the rotor of the electric motor, in that a position measuring device having a position sensor for measuring the rotor position is provided, in that the measuring signal output of the position measuring device and/or the terminal for the standard value signal are connected with the outputs of at least one comparison device the output of which is connected with a shut off device for shutting the electric motor off when a specified maximum value is exceeded by the position measuring signal and/or the standard value signal for the commutating position and/or when a difference lying outside a specified tolerance range arises between the standard value signal and the measuring signal of the position measuring device.
Advantageously, the existing standard value signal for the commutating position of the electric motor rotor is used for monitoring the position of the rotor so that in addition to the measuring signal for the position of the rotor, a further signal enabling a monitoring of the rotor position is evaluated. In this way, the invention makes use of the knowledge that the rotor of the electric motor can only rotate if the current or voltage vector applied to the windings of the electric motor also rotates. On the basis of the commutating position, the position of the rotor can thus be inferred. It consequently represents a second channel for recording position in relation to the first channel consisting of the position measuring device. A costly and expensive second position measuring device can consequently be saved. Through the comparison of the two signals with each other and/or of at least one of the signals with the specified maximum value as well shutting off the electric motor in the event of an error, the invention provides a way to conduct twofold monitoring of rotor motion which makes possible a safe operation of the electric motor.